Most 9x50 finder scopes front aperture is threaded for a 2" filter (all mine have been in any case...) You could possibly screw in a light pollution filter to help out in that respect.. ?? You would also want to make a dewshield for the finder/guider too, as they will not only collect dew quickly, but with the objective showing, any nearby light will cause a loss in contrast and make acquiring guidestars difficult...
Cheers.
Alex.
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